The manufacture of microelectromechanical systems (MEMS) devices typically entails the deposition, patterning and etching of various structural and sacrificial material layers to produce, for example, very sensitive movable mechanical parts. By way of example, the sensitive moving mechanical parts are typically made in a silicon structural layer (e.g., polysilicon or silicon-germanium), the sacrificial material underlying these mechanical parts is typically silicon dioxide, and the etch-stop layer underlying this silicon dioxide sacrificial layer is typically silicon nitride and/or another silicon structural layer (e.g., polysilicon or silicon-germanium).
A chemical etching technique, typically referred to as vapor phase etching (VPE), has the ability to etch with relatively fine resolution and high aspect ratio. Thus, VPE can be suitable for the removal of the sacrificial material underlying the mechanical parts formed in a structural layer. Removal of the sacrificial material enables release of the mechanical parts, thereby making them movable in accordance with a particular design. A VPE process involves a chemical reaction between etchant gases to attack the silicon-based sacrificial material. That is, the material to be etched is dissolved at its surface in a chemical reaction with the gas molecules. One common vapor phase etching technology is silicon dioxide etching using vapor hydrogen fluoride (HF).
Unfortunately, vapor HF can attack the silicon nitride underlying the sacrificial material layer, producing an undesirable fluorinated silicon nitride compound. This fluorinated compound remains as a residue on the exposed surfaces of the released structure. Additionally, this fluorinated compound is unstable in moist air and can undergo volumetric expansion in the presence of water. Volumetric expansion of the residue can push against the moving mechanical elements and can damage them. Additionally, or alternatively, a violent reaction can occur when water is introduced to the fluorinated compound that displaces the moving mechanical elements away from the reaction further damaging the moving mechanical elements. And still further, this fluorinated compound residue can become contaminated during VPE processing with metal (for example, aluminum and/or copper) released from collateral etching of exposed metallization layers. This metal contaminated residue can be difficult to remove and can negatively affect device performance.
Accordingly, what is needed is methodology for effectively removing the fluorinated compound to improve MEMS device yield and reliability.